I actually think Biden could lose independents in 2024 and still win reelection. Dubya and Obama also lost them in 2004 and 2012, respectively. Seems like indies are anti-incumbent regardless of whether a Democrat or Republican is in office.
Eh, DeSantis would probably make it competitive, but certainly not 20%. However, Trump winning by 9% is absurd. Democrats won them by 2 in 2022, and many Rs were basically a proxy for Trump. If anything, I imagine the worst case scenario for Biden is losing Indies by a few %, but given DeSantis/Trump policies, I don't see how anything close to 20% is realistic.